Method for joining steel to aluminum alloy components or titanium alloy components, and turbochargers obtained by the method

ABSTRACT

Blanks or structural components of steel are joined to those of an aluminum alloy (a) or titanium alloy (b) by friction-welding with insertion of at least one transition layer of a ductile subgroup metal, in particular by applying, according to (a), a pure nickel layer to the steel by friction-welding and joining the surface of the nickel layer, after weld bead removal, to the aluminum alloy by friction-welding. According to (b), a copper layer is applied to the steel by friction-welding and a vanadium layer is applied to the titanium alloy by friction-welding. The copper and vanadium surfaces are, after weld bead removal, then joined to one another by friction-welding. In the case of steel which tends to form martensitic zones, a layer of austenitic steel is applied by friction-welding before the application of the copper layer. The process is especially suitable for joining the turbine wheel and shaft or fixing component for the shaft of a turbocharger.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for joining blanks or structuralcomponents of steel to those of aluminum alloy or titanium alloy, and itcomprises turbochargers obtained by the method.

2. Discussion of Background

The joining of different materials plays a considerable part in thehighly developed fabrication technology, since frequently differentproperties are demanded within one arrangement, which properties cannotbe obtained by a single material so that, depending on the manufactureand operating function, different materials are used for individualelements of a component or of a structure, in order to arrive at anoptimum of economical manufacture and mechanical property.

Thus, for example, according to European Patent Specification 0,129,311,aluminum or aluminum alloy is used, for reasons of manufacturingtechnology, for the compressor rotor of a turbocharger and, because ofthe good mechanical properties such as strength and toughness, toolsteel is provided for the shaft to be fitted on or for the fixingcomponent for the latter, and these are to be joined to one another byfriction-welding.

Some difficulties arise, however, in following this recommendationsince, although aluminum can be joined to steel by friction-weldingunder certain conditions, making a welded joint between steel andhardenable aluminum alloys causes great difficulties.

Likewise, friction-welded joints, capable of being loaded, between steeland titanium alloys have hitherto not been achieved.

SUMMARY OF THE INVENTION

Accordingly, one object of the invention is to develop a novel method,by means of which blanks or structural components of steel can be joinedto those of aluminum alloys or titanium alloys by friction-welding.

The method according to the invention, developed for this purpose,comprises joining the blanks or components to one another with insertionof at least one transition layer of a ductile subgroup metal by frictionweld-joining of the individual contact surfaces.

Owing to the use of ductile pure metal layers, thermal strains betweenthe partners of the joint can be compensated. The material systems onthe joint surfaces remain clear, and especially the formation of brittlephases in the boundary region is suppressed. Of course, attention ispaid, here to the metallurgical compatibility of the partners of thejoint and to the suitability for friction-welding.

The joining of steel to a titanium alloy is carried out especially byapplying a copper layer to the steel by friction-welding and applying avanadium layer to the titanium alloy by friction-welding and thenjoining the copper and vanadium surfaces, after weld bead removal, toone another by friction-welding.

The friction welds are advantageously made with exclusion of the ambientair under an inert gas, in vacuo or especially under a liquid such as,for example, a petroleum/mineral oil mixture, such as is used forspark-erosion machining.

For the joining, conventional friction-welding machines can be usedwhich run at fixed speeds of rotation or with infinite adjustment of thespeed of rotation. Friction for a certain time or to a certain depth arepossible modes of operation.

In the method according to the invention, conventional speeds ofrotation in the region of 500 -2000 rpm, especially in the range around1000 rpm, and forging pressures up to about 500 kN, especially in therange 100-200 kN, can be applied.

The interlayers in the finished joint are as thin as possible; theirthickness is advantageously within the mm range.

Before the friction-welding, the surface is carefully prepared;especially, the freshly turned surface is degreased in an ultrasonicbath, rinsed with alcohol and dried.

In order to avoid high starting torques, a conical joining surface canbe provided on one joining partner but, for reasons of the loadingcapacity of the joint, flat joining surfaces are preferred.

Roughnesses of the surfaces to be joined of 30-300 μm, especially about100 μm, are appropriate.

Special features of the invention can be taken from the subclaims andfrom the following description by reference to examples.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIGS. 1 to 5 show the result of friction-welded joints according toExample 1;

FIGS. 6 and 7 show macrophotographs of the joints according to Example2; and

FIGS. 8 and 9 show a compressor rotor with turbine wheel and shaft of aturbocharger in two alternatives for the fixing component for the shaft.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1 Joining of Quenchedand Tempered Steel St575 With Hardenable Aluminum Alloy AN40.

A nickel platelet (2×50×50) was applied by friction-welding to the endface, prepared as indicated above, of a cylindrical steel bar (φ45×140).After weld bead removal from the end f ace of the welded-on nickel layeron the lathe, a cylindrical rod of the aluminum alloy AN40 (φ50×150) wasfitted on in a second friction-welding pass, and the St575/Ni/AN40 jointwas completed in this way. The strength reached was 211 N/mm². Thestrength of the joint is affected by the surface roughness of the nickellayer. The highest strength values were reached with roughness values ofNi bonding surface (to the AN40) of R_(Z) =162 μm, R_(A) =43 μm andR_(t) =185 μm. The highest strength is here observed on a joint withAN40 in the hardened state.

The attached figures show the friction-welded blank (FIG. 1) after weldbead removal (FIG. 2), and the cross-section through the blank (FIG. 3).FIGS. 4 and 5 show the microstructure of the transitions in this joint.

In order to avoid embrittlements in the bonding plane, the St575/Nijoint was heat-treated for 60 minutes in a high vacuum at 680° C.

In a manner analogous to that described above, blanks of otherhardenable aluminum alloys such as AlCuMg1, AlCuMg2 or AlCuMg1828 can bejoined to steel by friction-welding with an Ni interlayer.

Example 2 Joining of Quenched and Tempered Steel St575 to Titanium AlloyTi6Al4V.

On the one hand, a vanadium platelet was applied by friction-welding toa round bar of the titanium alloy (φ45×150) and, on the other hand, acopper layer was applied by friction-welding to a round St575 bar(φ45×140), and the weld beads were carefully removed on the lathe. Avanadium layer of 3.5-4.5 mm thickness remained on the titanium, and acopper layer of 5-8.5 mm thickness remained on the St575. The furtherwelding of vanadium to copper led to a shortening at the expense of thecopper. In the finished joint, the copper layer had a thickness of lessthan 1 mm.

Particularly good load-bearing capacities of the Ti6A14V/V/Cu/St575joint up to the range of 660 N/mm² is obtained by high forging pressuresin the range of 150 N/mm² during the friction-welding.

In order to avoid increases in the hardness of the St575, an interlayerof austenitic X1OCrNiTi189 steel was incorporated between the copper andSt575 in further tests. This additional interlayer does not undergo anystructural changes during friction-welding, but it prevents heating ofthe St575 into the austenite region and the associated hardness increaseduring cooling. The X1OCrNiTi189 layer was friction-welded to the St575and this joint was then heat-treated for one hour at 600° C. in order toeliminate the hardness increase in the St575. After removal of weldbeads of the austenitic layer on the lathe to a 2.5 mm thickness, itcould be further friction-welded to copper. With such a joint, strengthsof 454 N/mm² were reached.

FIGS. 6 and 7 show macrophotographs of the joints.

With the same arrangement of interlayers and the same processing steps,material joints of steel to α- and (α+β)-titanium alloys such as, forexample, TiCu2 or TiA15Fe2.5, can be made.

Steels of low carbon content, which do not tend to form martensite inthe course of cooling after friction-welding, can be directlyfriction-welded to copper, omitting an interlayer of the X1OCrNiTi189steel.

Example 3

The method indicated above in Example 1 was used for joining a fixingcomponent for the shaft of a turbocharger to the compressor rotor:

The component thus produced is illustrated on the attached FIG. 8: thefixing component 16 provided with a thread is friction-welded-viainterlayers 18 to the axial end face 17 of the hub 11 joined to blades12. The shaft 21 integral with the turbine disk 26 with integral orattached blades 27 is screwed to the fixing component 16, and acylindrical or stepped ring 23, which is shrunk on or slipped on andreceives the radial sealing rings 24 and is axially formed as a slidingsurface 25, grips over the shaft 21.

Instead of the fixing component 16, the shaft 21 itself can also befriction-welded to the hub 11. The same applies to the connection of theturbine wheel 26 to the shaft 21.

Alternatively, the compressor rotor 11-17 can also consist of a wroughttitanium alloy instead of the wrought aluminum alloy and is then joinedaccording to Example 2 by friction-welding to the fixing component 16 orto the shaft 21.

Obviously, numerous modifications and variations of the presentinvention are possible in the light of the above teachings. It istherefore to be understood that, within the scope of the appendedclaims, the invention may be practiced otherwise than as specificallydescribed herein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A method for joining components of steel tothose of a titanium alloy, comprising applying a copper layer to thesteel by friction-welding and applying a vanadium layer to the titaniumalloy by friction-welding, removing a weld bead formed and joining thecopper and vanadium surfaces to one another by friction-welding.
 2. Themethod as claimed in claim 1, comprising applying a layer of austeniticsteel by friction-welding to said steel, which tends to form martensiticzones, before applying the copper layer.